This invention relates to coherent imaging systems using vibratory energy and, in particular, to phased array sector scanning ultrasound imaging systems.
There are a number of modes in which vibratory energy, such as ultrasound, can be used to produce images of objects. The ultrasound transmitter may be placed on one side of the object and the sound transmitted through the object to the ultrasound receiver placed on the other side ("transmission mode"). With transmission mode methods, an image may be produced in which the brightness of each pixel is a function of the amplitude of the ultrasound that reaches the receiver ("attenuation" mode), or the brightness of each pixel is a function of the time required for the sound to reach the receiver ("time-of-flight" or "speed of sound" mode). In the alternative, the receiver may be positioned on the same side of the object as the transmitter and an image may be produced in which the brightness of each pixel is a function of the amplitude or time-of-flight of the ultrasound reflected from the object back to the receiver ("refraction", "backscatter" or "echo" mode). The present invention relates to a backscatter method for producing ultrasound images.
There are a number of well known backscatter methods for acquiring ultrasound data. In the so-called "A-scan" method, an ultrasound pulse is directed into the object by the transducer and the amplitude of the reflected sound is recorded over a period of time. The amplitude of the echo signal is proportional to the scattering strength of the refractors in the object and the time delay is proportional to the range of the refractors from the transducer. In the so-called "B-scan" method, the transducer transmits a series of ultrasonic pulses as it is scanned across the object along a single axis of motion. The resulting echo signals are recorded as with the A-scan method and either their amplitude or time delay is used to modulate the brightness of pixels on a display. With the B-scan method, enough data are acquired from which an image of the refractors can be reconstructed.
In the so-called C-scan method, the transducer is scanned across a plane above the object and only the echoes reflecting from the focal depth of the transducer are recorded. The sweep of the electron beam of a CRT display is synchronized to the scanning of the transducer so that the x and y coordinates of the transducer correspond to the x and y coordinates of the image.
Ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements sandwiched between a pair of electrodes. Such piezoelectric elements are typically constructed of lead zirconate titanate (PZT), polyvinylidene difluoride (PVDF), or PZT ceramic/polymer composite. The electrodes are connected to a voltage source, and when a voltage is applied, the piezoelectric elements change in size at a frequency corresponding to that of the applied voltage. When a voltage pulse is applied, the piezoelectric element emits an ultrasonic wave into the media to which it is coupled at the frequencies contained in the excitation pulse. Conversely, when an ultrasonic wave strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. Typically, the front of the element is covered with an acoustic matching layer that improves the coupling with the media in which the ultrasonic waves propagate. In addition, a backing material is disposed to the rear of the piezoelectric element to absorb ultrasonic waves that emerge from the back side of the element so that they do not interfere. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684, 4,425,525, 4,441,503, 4,470,305 and 4,569,231, all of which are assigned to the instant assignee.
When used for ultrasound imaging, the transducer typically has a number of piezoelectric elements arranged in an array and driven with separate voltages (apodizing). By controlling the time delay (or phase) and amplitude of the applied voltages, the ultrasonic waves produced by the piezoelectric elements (transmission mode) combine to produce a net ultrasonic wave focused at a selected point. By controlling the time delay and amplitude of the applied voltages, this focal point can be moved in a plane to scan the subject.
The same principles apply when the transducer is employed to receive the reflected sound (receiver mode). That is, the voltages produced at the transducer elements in the array are summed together such that the net signal is indicative of the sound reflected from a single focal point in the subject. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each transducer array element.
This form of ultrasonic imaging is referred to as "phased array sector scanning", or "PASS". Such a scan is comprised of a series of measurements in which the steered ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received and stored. Typically, the transmission and reception are steered in the same direction (.theta.) during each measurement to acquire data from a series of points along a scan line. The receiver is dynamically focused at a succession of ranges (R) along the scan line as the reflected ultrasonic waves are received. The time required to conduct the entire scan is a function of the time required to make each measurement and the number of measurements required to cover the entire region of interest at the desired resolution and signal-to-noise ratio. For example, a total of 128 scan lines may be acquired over a 90 degree sector, with each scan line being steered in increments of 0.70.degree.. A number of such ultrasonic imaging systems are disclosed in U.S. Pat. Nos. 4,155,258, 4,155,260, 4,154,113, 4,155,259, 4,180,790, 4,470,303, 4,662,223, 4,669,314 and 4,809,184, all of which are assigned to the instant assignee.
The ability of current ultrasonic imaging systems to steer and dynamically focus the ultrasonic energy while in the receive mode far exceeds the ability to form beams, steer them and dynamically focus them while in the transmit mode. For example, in U.S. Pat. No. 4,886,069 which issued on Dec. 12, 1989 and is entitled "Method Of, And Apparatus For Obtaining A Plurality Of Different Return Energy Imaging Beams Responsive to a Single Excitation", I disclose a method for forming two separate, steered and dynamically focused beams from each received ultrasonic echo signal. Thus, a single ultrasonic transmission may be steered in a particular direction (.theta.) and focused at a particular range (R) with the simultaneous production of two dynamically focused receive beams disposed on each side of the transmit beam angle (.theta.) by an amount .+-..DELTA..theta.. This enables the total scan time to be reduced because fewer pulse transmissions are now required to produce the same number of received beams.
The inability to precisely coordinate transmit beam steering with receive beam steering and the inability to dynamically focus the transmit beam results in a reduction in signal-to-noise ratio and resolution in the reconstructed image. It can be appreciated that a peak signal will be produced when the echo signal is received from reflectors which are located along the transmit beam direction (.theta.). By forming multiple receive beams using the teachings of the above-cited patent, however, the receiver beam angle is offset by an angle .+-..DELTA..theta. from this transmit beam angle (.theta.) with a consequent reduction in signal strength. Similarly, the transmit beam sharply focuses the ultrasonic energy on reflectors located at a particular range (R), while the receiver dynamically changes its focus over a relatively large range of values as the ultrasonic echo signal is received. Where the dynamic focus of the receiver corresponds to the fixed focus of the transmitter, the image quality is best.